1. Field of the Invention
This invention relates to semiconductor device electrical test systems, and more particularly to semiconductor device electrical test systems employing probe cards with removable probe heads.
2. Description of Related Art
A wafer fabrication process typically forms multiple identical integrated circuits upon and within frontside surfaces of each of several semiconductor wafers processed as a group (i.e., lot). Each individual integrated circuit is formed within a die area of one of the semiconductor wafers. Following wafer fabrication, the dice are subjected to xe2x80x9cwafer sortxe2x80x9d electrical testing, then separated from the wafers. Fully functional die are typically packaged and sold as individual semiconductor devices.
Wafer sort electrical testing is performed to ensure each integrated circuit functions properly according to a predetermined electrical specification. Following wafer fabrication, each wafer is typically placed upon a flat surface of a test system with the frontside surface facing up. A wafer under test is raised up until bonding pads of one of the integrated circuits contact many fine wires or xe2x80x9ctest probesxe2x80x9d of the test system. The test system may provide electrical power to the integrated circuit via some of the test probes, and may provide input signals and/or receive output signals from the integrated circuit via other test probes. The wafer under test is lowered, repositioned, and raised again until all of the integrated circuits formed on the wafer under test have been tested.
A modem integrated circuit may have several hundreds of very small bonding pads arranged within a rectangular die area having a surface area of about half a square inch. As a result, several hundreds of closely spaced test probes may be required to carry out the electrical testing during wafer sort. The fine wire test probes are delicate and easily damaged. Further, they are typically uninsulated, and must remain spaced apart to prevent shorting and to reduce inter-probe capacitance.
A typical modem test system includes a xe2x80x9cprobe cardxe2x80x9d which functions as an electrical interface between the test system and the device under test. The probe card effects a transition from relatively large electrical conductors (i.e., wires) used within the test system to the fine wire test probes which contact the bonding pads of the device under test. FIG. 1 is a bottom plan view of a typical probe card 10 which includes a circular printed circuit board (PCB) 12 having two opposed surfaces. Probe card 10 may be, for example, a COBRA(copyright) probe card (Wentworth Laboratories, Brookfield, Conn.). One surface of PCB 12 is connected to a metal mounting ring 14 about a periphery. An alignment ring 16 and an interface circuit 18 are connected to the other surface of PCB 12. A removable probe head assembly 20 including a probe head body 22 is adapted for mounting within alignment ring 16 as shown.
FIG. 2 is a sectional view of typical probe card 10 as indicated in FIG. 1. Probe head body 22 has a pair of opposed surfaces and a set of probe holes extending between the opposed surfaces. The probe holes are arranged according to a pattern defined by electrical contacts 26 on a surface of a device under test 28. Electrical contacts 26 may include solder bumps formed upon flat metal pads as shown in FIG. 2.
Probe head assembly 20 includes a set of contact probes 24 each having two ends. Each contact probe 24 extends through a probe hole in probe head body 22 such that the ends of contact probe 24 project outwardly from the opposed surfaces of probe head body 22. A pair of alignment pins 25a-b protruding from probe head body 22 fit into corresponding holes in alignment ring 16 when probe head body 22 is coupled to alignment ring 16.
PCB 12 has two sets of bonding pads: a first set of bonding pads 29 on the surface of PCB 12 to which mounting ring 14 is connected, and a second set of bonding pads 30 on the opposite surface of PCB 12 adjacent to interface circuit 18. PCB 12 includes electrically conductive metal traces, insulated by fiberglass-epoxy dielectric material, which interconnect bonding pads 29 and bonding pads 30. Bonding pads 29 are connected to the test system, and bonding pads 30 are connected to corresponding bonding pads of interface circuit 18.
Interface circuit 18 has a set of bonding pads 32 on one of two opposed surfaces, and a set of contact pads 34 on the other of the two opposed surfaces. Bonding pads 32 are located on a surface of interface circuit 18 adjacent to PCB 12, and are connected to corresponding bonding pads 30 of PCB 12. Contact pads 34 are electrically conductive flat metal pads formed flush with a surface of interface circuit 18 opposite PCB 12. Contact pads 34 are arranged according to the pattern defined by electrical contacts 26 on the surface of device under test 28. Interface circuit 18 includes electrically conductive metal traces, typically insulated by a ceramic dielectric material, which interconnect bonding pads 32 and contact pads 34.
During testing, probe head body 22 is brought proximate the surface of device under test 28 such that one end of each contact probe 24 contacts a corresponding electrical contact 26 on the surface of device under test 28. When probe head body 22 is properly aligned with and mechanically coupled to interface circuit 18, the other end of each contact probe 24 contacts the corresponding contact pad 34 of interface circuit 18. Electrically conductive signal paths are formed between the test system and contact probes 24 via PCB 12 and interface circuit 18. The test system provides electrical power and input signals destined for device under test to bonding pads 29 on PCB 12, and also monitors output signals produced by device under test 28 via bonding pads 29. The electrical power, input signals, and output signals are conveyed to and from device under test 28 via electrical signal paths formed through PCB 12, interface circuit 18, and contact probes 24 of probe head assembly 20.
Correct operation of the test system requires proper alignment between probe head body 22 and interface circuit 18. As described above, interface circuit 18 is firmly attached to PCB 12. Screw fasteners are typically used to attach alignment ring 16 to PCB 12, and to attach probe head assembly 20 to alignment ring 16. Subject to mechanical wear and tear, probe head assemblies are routinely removed from probe cards for maintenance and repair.
A problem arises when attaching probe head assembly 22 to alignment ring 16 following maintenance or repair. The mechanical tolerances associated with the screw fasteners are great enough to require tedious and time consuming manual adjustment of the alignment between alignment ring 16 and PCB 12, as well as the alignment between probe head assembly 20 and alignment ring 16, in order to attain correct test system operation. It is noted that alignment pins 25a-b reduce, but do not eliminate, the mechanical alignment problem. As probe head body 22 is typically opaque, visual verification of alignment between probe head body 22 and interface circuit 18 is made impossible, essentially reducing the alignment process to one of trial and error.
It would thus be desirable to have a probe card which provides for alignment of the probe head body directly to the interface circuit, and an associated alignment method. Such an apparatus and method would eliminate the required tedious and time consuming manual adjustment of the alignment between alignment ring 16 and PCB 12, and between probe head assembly 20 and alignment ring 16, in order to attain correct test system operation following replacement of a probe head assembly.
The problems outlined above are in large part solved by an electrical test probe card including a removable probe head assembly with alignment features, and a method for aligning the probe head assembly to the remainder of the probe card. The probe card includes a removable probe head assembly used to provide an electrical interface between an interface circuit and a semiconductor device under test. The probe head assembly includes a probe head body having a pair of opposed surfaces. A set of probe holes and a set of alignment holes extend between the opposed surfaces. Each probe hole is adapted to receive an electrically conductive probe, and each alignment hole is adapted to pass optical illumination used to align the probe head body with the interface circuit.
The probe holes are arranged according to a first pattern defined by a set of electrical contacts on a surface device under test. A surface of the interface circuit includes a set of electrical contacts arranged according to the first pattern, and a set of alignment marks defining a second pattern. Each probe hole thus has a corresponding electrical contact on the surface of the device under test, and a corresponding electrical contact on the surface of the interface circuit. The alignment holes are arranged according to the second pattern such that each alignment hole has a corresponding alignment mark on the surface of the interface circuit.
The probe head assembly also includes a set of contact probes each having two opposed ends. Each contact probe extends through a probe hole such that the ends of the contact probes project outwardly from the opposed surfaces of the probe head body. During testing, each contact probe acts as an electrical conductor between a corresponding electrical contact on the surface of the device under test and a corresponding electrical contact on the surface of the interface circuit.
During attachment of the probe head assembly to the remainder of the probe card, one of the pair of opposed surfaces of the probe head body is brought into contact with the surface of the interface circuit including the electrical contacts and the alignment marks. When an end of each contact probe contacts the corresponding electrical contact on the surface of the interface circuit, the probe head body is properly aligned with the interface circuit, and the optical illumination passing through any member of the set of alignment holes illuminates the corresponding alignment mark upon the surface of the interface circuit.
The electrical contacts on the surface of the interface circuit are preferably grouped toward the center of the surface, and the alignment marks are preferably positioned between the electrical contacts and a periphery of the surface. For example, the surface of the interface circuit may be rectangular, having two pairs of opposite corners, and the electrical contacts may be grouped toward the center of the surface. In this case, the alignment marks are preferably positioned in opposite corners of the surface of the interface circuit.
The alignment marks may advantageously be electrical contacts formed simultaneously with, and having physical dimensions identical to, the set of electrical contacts which interface with the contact probes. The electrical contacts and the alignment marks may be, for example, round metal pads. In this case, the alignment holes may be cylindrical and have a diameter greater than the diameter of the alignment marks. This configuration would allow an operator to center the alignment marks in circular ends of the alignment holes, thus aiding in the visual alignment process.
The present method for ensuring electrical contact between the contact probes of the probe head assembly and the electrical contacts of the interface circuit includes bringing one surface of the probe head body into contact with the surface of interface circuit such that the surface of the interface circuit closes a first end of each of the alignment holes. A second end of each of the alignment holes opposite the surface of the interface circuit remains open. One of the alignment holes is then selected. Optical illumination (e.g., light) is directed into the open end of the selected alignment hole. A portion of the optical illumination reflected from the surface of interface circuit at the closed end of the selected alignment hole and exiting the open end of the selected alignment hole is visually detected by an operator""s eye. Such visual detection may be aided by a magnifying lens or microscope. The surface of the probe head body in contact with the surface of the interface circuit is slid along the surface of the interface circuit until the alignment mark corresponding to the selected alignment hole is visible at the closed end of the selected alignment hole. For example, a circular alignment mark may be centered in the larger circular cross section of a cylindrical alignment hole. The above selecting, directing, detecting, and sliding steps may be repeated until all of the alignment holes have been selected and the corresponding alignment marks are visible at the closed ends of all of the alignment holes. The position of the probe head body relative the interface circuit may then be fixed.